Of course there is a gap between what is optimum and what is reasonble to achieve.
For example, you could check them once a year or stop every few miles during your trip to check them. Neither of these are reasonable!
I go by checking them each time the expected climate is changing or during periodic rotation of tyres.
For me, its reasonable this way because the climate in my area is quite mild and does not change drastically throughout each season. So I am able to adjust a few PSI as the seasons change. If I do it more often, I find I am checking and finding all the pressures are correct. I also check them before any long road trip. I do visual checks during cleaning.
In your case, I would prefer to adjust them for a 20C change in temperature, even if it means doing it weekly. Make it as easy as possible for yourself, have the air compressor and gloves ready in the vehicle so that its as convenient as possible to do routinely.
Otherwise, you may decide a compromise pressure will get you through 0C and 20C, this would a PSI number that gets you close enough during both expected temperatures. (I don't encourage this way, just adding for information.)
Update based on comment
This website claims: "under inflated by 15 psi (1 bar) lead to 6% greater fuel consumption."
Another website also has some figures for a particular Honda Accord:
- "6 psi below the recommended spec suffers a 5 percent decrease in fuel economy."
- "inflated 6 psi below spec, would wear out 25 percent faster."
- "tire pressure monitoring systems are designed to alert the driver only if a tire's pressure is 25 percent or more below specification, which is already well into the danger zone."
My advice is, no compromise on a safety item is acceptable.